Allow to select and see the diff between two commits

[guillep]

From mailing list

I see here you show two tabs with fixed diffs:

• Blue selection to HEAD, and;
• Blue selection to previous
  Now asking for the moon, what I'd like to try is a second selection in another colour (e.g. purple),
  that perhaps you move holding the ALT key, that would select any two commits to diff.
  And also experiment with showing that second selection as either:
• holding down the ALT key to toggle the existing commit list in place to show the purple selection
• since many of us have wide screen, have a second commit list to the right of the existing commit list
• have a second commit list below the existing commit list
• flexibility of user changing between those three